1. Field of the Invention
The present invention relates to a wire bonding method, wire bonding apparatus and wire bonding program, and more particularly relates to a wire bonding method, wire bonding apparatus and wire bonding program that controls the formation of a wire loop between a first bonding point and a second bonding point.
2. Prior Art
A wire bonding apparatus is an apparatus that connects, by a slender metal wire, a first bonding point such as an input-output terminal, etc. on a semiconductor chip such as an LSI, etc. and a second bonding point such as a terminal on the circuit board on which the semiconductor chip is mounted. In this wire bonding, it is desirable that the first bonding point and the second bonding point be connected by a wire loop that has an appropriate shape. If the height of the wire loop is too high, the size of the package, etc. increases, and drooping of the wire tends to occur. If the wire loop is too low, there is a danger that the wire will contact the semiconductor chip or wiring pattern. Accordingly, the operation of wire bonding apparatuses is controlled in order to form an appropriate wire loop.
FIG. 4 shows the procedure of a wire loop formation method of prior art.
In this method, the wire 12 is supplied to a capillary 14 from a wire spool; and during the supply of the wire, an appropriate tension is applied to the wire using air. The capillary 14 is a tubular member; and the wire 12 passes through the hollow portion of this capillary 14 and is held by a wire damper 16 installed above the capillary 14. The wire damper 16 clamps and releases the wire and is moved together with the capillary 14. The objects of bonding 18, 19 are held on a carrying stand (not shown). In order to form a wire loop between the first bonding point 20 of the object of bonding 18 and the second bonding point 21 of the object of bonding 19, the capillary 14 is moved in relative terms with respect to the objects of bonding held on the carrying stand by the processes shown by steps (a) through (g) of FIG. 4, so that necessary kinks necessary for a wire loop are formed in the wire 12.
More specifically, step (a) is the process that connects the wire 12 to the object of bonding 18. In this process, the capillary 14 is lowered with the damper 16 in an open state, so that a ball formed beforehand on the tip end of the wire is bonded to the first bonding point 20.
Next, in steps (b), (c) and (d), the capillary 14 is raised slightly, caused to move horizontally in the opposite direction from the second bonding point, and then raised again. As a result, kinks 22 and 24 are formed in the wire 12. The reason that the kink 22 is formed is that the wire is hardened when the ball is formed on the tip end of the wire, so that the portion of the wire that is located within a certain range from the first bonding point 20 is difficult to bend. Also, the portion of the wire that extends from the first bonding point 20 to the kinks 22 and 24 corresponds to the portion called the “neck portion”, where the wire 12 is caused to stand up in the vicinity of the first bonding point when the wire loop is formed.
After the capillary 14 is raised in step (d), the capillary 14 is moved horizontally in the opposite direction from the second bonding point as shown in step (e). As a result, a kink 26 is formed.
Next, in step (f), the capillary 14 is caused to move in the horizontal direction while being raised, until the capillary 14 arrives at a point above the first bonding point 20. As a result, a kink 28 is formed. The amount by which the capillary is raised in this case is set so that the length of the wire that is paid out from the capillary 14 between the first bonding point 20 and the kink 28 corresponds to the length of the wire loop from the first bonding point 20 to the second bonding point 21.
In this step (f), when the capillary 14 is raised to a specified height and the kink 28 is formed, the wire damper 16 clamps the wire 12. In other words, no wire 12 is paid out even if the capillary 14 is moved. Keeping this clamping state, the capillary 14 is moved to the second bonding point 21 of the object of bonding 19 by a circular arc movement or by a lowering movement after a circular arc movement in step (g). In this case, the kink 28 of the wire 12 is positioned at the second bonding point 21, and bonding is performed there.
With the steps described above, a wire loop 30 that has kinks 22, 24 and 26 is formed between the first bonding point 20 and the second bonding point 21. However, since the kinking of the kink 28 is weak, and since the interval between the kink 26 and kink 28 must be long in order to ensure that the wire loop 30 has a certain height, the wire loop 30 tends to droop at the second bonding point 21.
FIG. 5 is a diagram showing the procedure used in another wire loop formation method of prior art. In this method, the wire loop can be endowed with a greater rise in the vicinity of the second bonding point than in the method shown in FIG. 4. In this FIG. 5, the steps from (a) to (c) are processes in which a so-called neck portion is formed in the same manner as in the method of FIG. 4.
After the capillary 14 is moved horizontally in the opposite direction from the second bonding point in step (c) in FIG. 5, the amount by which the capillary 14 is raised in step (d) is greater than in the corresponding step of FIG. 4. Furthermore, as seen from step (e), the amount by which the capillary 14 is moved horizontally in the opposite direction from the second bonding point is also greater than the corresponding step of FIG. 4. Accordingly, the kink 36 that is formed here is positioned further from the first bonding point 20, i.e., closer to the second bonding point, than the corresponding kink 26 in the method of FIG. 4.
Next, in step (f), the capillary 14 is moved in the horizontal direction while being raised, so that the capillary 14 arrives at a point above the first bonding point 20. As a result, a kink 38 is formed. The amount by which the capillary is raised in this case is set so that the length of the wire paid out from the capillary 14 between first bonding point 20 and the kink 38 corresponds to the length of the wire loop from the first bonding point 20 to the second bonding point 21. Since a considerable amount of wire is paid out in step (d), the amount by which the capillary is raised in step (f) is small. Accordingly, the interval between the kink 36 and kink 38 is short, and the shape of the wire in this interval takes a circular arc.
When the capillary 14 is raised to a specified height and the kink 38 is formed in step (f), the wire clamper 16 clamps the wire 12 so that the paying out of the wire is stopped. Keeping this clamping state, the capillary 14 is moved to the second bonding point 21 of the object of bonding 19 by a circular arc movement or by a lowering movement after a circular arc movement as seen in step (g). In this case, the kink 38 of the wire 12 is positioned at the second bonding point 21, and bonding is performed there.
With the steps above, a wire loop 40 that has kinks 22, 24 and 36 is formed between the first bonding point 20 and the second bonding point 21. In this case, the kink 36 is close to the second bonding point 21, and the shape between the kink 36 and kink 38 is a circular arc. Accordingly, the wire loop can be endowed with a greater rise in the vicinity of the second bonding point than in the method shown in FIG. 4.
The above methods are described in, for instance, Japanese Patent Application Laid-Open (Kokai) Nos. S63-42135 (page 2, FIGS. 1 and 2), H04 –318943 (pages 3 –4, FIG. 2) and H10-189641 (pages 2-4, FIGS. 1-7).
However, in the above conventional wire loop formation methods, the kinking of the kink at the second bonding point is week, and the rising portion of the wire loop at the second bonding point tends to droop to some extent. Accordingly, there is a danger that the wire loop may contact the object of bonding. In cases where the second bonding point is a bonding lead on a circuit board, etc., even if the wire loop droops in the vicinity of the second bonding point, the wire loop will merely contact the bonding lead to which the wire is to be bonded, and there would be no problem. On the other hand, in cases where the second bonding point is on the surface of an element such as an LSI, etc. or in cases involving a compact or thin package in which the wire loop is extremely low, there is a danger that drooping of the wire loop in the vicinity of the second bonding point may lead to unexpected short-circuiting.
Furthermore, if drooping of the wire loop results in contact with the object of bonding when the capillary, holding the wire, is being moved to the second bonding point in a circular-arc movement, etc., there is a danger that the shape of the wire loop is deformed by the rebound movement. Furthermore, there is a possibility that the wire loop may fall over in the vicinity of the second bonding point, and there is also a danger that a portion of the wire loop may be pushed into the interior of the hollow portion of the capillary.
In addition, in the prior art methods, when the wire is paid out to the position of the kink (28 in FIG. 4, 38 in FIG. 5) that corresponds to the second bonding point, the capillary is stopped at a specified height, and the paying out of the wire is stopped by clamping the wire so that this portion of the wire is formed into a kink. Accordingly, the total length of the wire and the length of the standing portion in the vicinity of the second bonding point are unstable. The reason for this is that there is friction between the hollow portion of the capillary and the wire, and there is also variation in the air tension that applies an appropriate tension to the wire; as a result, even if the capillary is stopped at a specified height, the amount of wire that is paid out is not constant.
As seen from the above, in wire loop formation methods of the prior art, there is a danger that the wire loop will contact the object of bonding, and the height and shape of the wire loop cannot be stably formed.